Lesson Title: Calculating Empirical and Molecular Formulas
State Standards: GLEs/GSEs
PS2 (Ext)-6
Students demonstrate an understanding of physical, chemical, and nuclear changes by... 6aa-using chemical equations and information about molar masses to predict quantitatively the masses of reactants and products in chemical reactions.
National Standards:
Context of Lesson:
This is a 90 minute lesson during which students will explore the methods necessary to calculate an empirical formula. They will then be introduced to the methods required to obtain a molecular formula from an empirical formula and be presented with problems that will connect the two concepts. It should be impressed upon students that being able to calculate a molecular formula is a very useful tool when it comes to Chemistry. The lesson can potentially run over into a second 90 minute period depending on how well students are able to comprehend the material.
Opportunities to Learn:
Depth of Knowledge
Prerequisite Knowledge
Percent composition
Using the mole
Determining the molar mass of a compound
Plans for Differentiating Instruction
Students who may have difficulty with calculating empirical and molecular formulas should be given multiple examples of each to reinforce the concept presented. I am not convinced that this is sufficient. The students who are struggling are not the ones who will do extra work. Will you offer other supports? What about the students who learn quickly?
Accommodations and modifications
Environmental factors
Materials
Students will need all required materials (textbook, notebook, calculator, pen/pencil) Will student really use their textbooks in class? Do you remember how annoying it is to carry a textbook around that is never used?
Objectives:
Students will derive the empirical formula and the molecular formula from experimental data. Anything else?
Instruction:
Opening:
Students will first be administered a brief quiz (mainly conceptual) on the reading assignment that they were asked to complete for homework at the end of the previous class:
The answers to the quiz will be embedded in the lesson and it is not necessary to take separate time to go over the quiz; however, the answers should be pointed out where they appear in the lesson. How will your students know this? What question will you ask to frame the day's activities?
Engagement:
Explain to students that percent composition data can now be used to calculate what we call the empirical formula, which we define as: a formula which gives the lowest whole-number ratio of the atoms of the elements in a compound. From the empirical formula we are able to calculate the molecular formula, which we define as: a simple multiple of the empirical formula. Students should record the following examples into their notes. I find this confusing. Will you use a visual representation to illustrate the difference (and utility) of each type of formula?
For example: Dinitrogen tetrahydride, whose molecular formula is N2H4, has an empirical formula of NH2 because this is the simplest ratio of nitrogen to hydrogen in the compound.
Now allow students to investigate: Welders use ethyne (C2H2) in their welding torches. Styrene (C8H8) is used in making polystyrene - which are the little beads used in packaging. Do both of these molecular formulas have the same empirical formula? If so, what is it? Answer: Yes, CH
Now, how do we go about calculating the empirical formula of a compound from the percent composition? Let's look at the following example to answer our question: What is the empirical formula of a compound that is 25.9% N and 74.1% O?
First, we must understand that if we have 100 g of the compound we will have 25.9 g of N and 74.1 g of O. Does everyone see why? Don't give answers, ask questions. They should be able to do this based on previous instruction, right? If you provide the answer, you are making their previous learning unnecessary.
Second, convert these values to moles: 25.9 g N x 1 mol N/14.0 g N = 1.85 mol N and 74.1 g O x 1 mol O/16.0 g O = 4.63 mol O
Third, the correct values can be obtained by dividing both molar quantities (N and O) by the smaller number of moles (in this case the moles of N):
1.85 mol N/1.85 = 1 mol N and 4.63 mol O/1.85 = 2.5 mol O So we have an empirical formula of N1O2.5, is this an acceptable answer? Why or why not? Answer: No, not a whole-number ratio
Fourth, this formula does not represent the lowest whole-number ratio. So what should we do? Answer: multiply each part of the ratio by a number that converts the fraction 2.5 to a whole number. In this case we multiply by 2: 1 mol N x 2 = 2 mol N and 2.5 mol O x 2 = 5 mol O So, we get: N2O5 for our empirical formula.
Is there a way to communicate an overall goal and how the students figure out these steps based on their previous work? The problem is that if you do it, it's just transmission, but if they do it, its knowledge construction.
Assign the following practice problem:
Calculate the empirical formula of each compound:
94.1% O, 5.9% H
67.6% Hg, 10.8% S, 21.6% O
While students are working on these move around the room to monitor their progress. Once they are finished have volunteers put the solutions up on the board and explain their answers.
Let's now move on to molecular formulas. Why? Is there a question / puzzle/ challenge you can present to frame this procedure. What is the goal? Why is this important? What are we learning that is new? The molecular formula of a compound is either the same as its experimentally determined empirical formula, or it is a simple whole-number multiple of it. We can determine the molecular formula of a compound if we know two things, what are they? Answer: Its empirical formula and its molar mass. Give students the following practice example: Calculate the molecular formula of the compound whose molar mass is 60.0 g and empirical formula is CH4N.
First, we determine the molar mass of the empirical formula. For example: CH4N has an empirical formula mass of 30 g
Second, divide the empirical formula mass into the molar mass: molar mass = 60 g, empirical formula mass = 30 g 60.0 g/30.0 g = 2
Third, multiply the formula subscripts (from the empirical formula) by this value to get the molecular formula: 2(CH4N) = C2H8N2
Assign the following practice problem:
Find the molecular formula of each compound given its empirical formula and molar mass:
Ethylene glycol (CH3O), which is used in antifreeze, molar mass = 62 g/mol
p-Dichlorobenzene (C3H2Cl), moth balls, molar mass = 147 g/mol
Closure:
Ask students the following questions to close the lesson:
How do we calculate empirical formulas? i.e. What steps do we follow?
Why is it useful to do so?
How do we calculate molecular formulas? i.e. What steps do we follow?
Provided that students were able to get through the entire lesson, assign section review 7.3 on p. 195 for homework, problems: 39, 41a, 42a, 43. Explain to students that this will be checked and gone over in the beginning of the next class and that shortly after to expect a brief quiz (mainly mathematical ). Good.
Assessment:
During the next class period, after the section review 7.3 homework assignment has been collected, checked, and gone over, administer the following quiz to sumatively assess students understanding on the calculation of empirical and molecular formulas:
Lesson Plan
Lesson Title: Calculating Empirical and Molecular Formulas
State Standards: GLEs/GSEs
PS2 (Ext)-6Students demonstrate an understanding of physical, chemical, and nuclear changes by...
6aa-using chemical equations and information about molar masses to predict quantitatively the masses of reactants and products in chemical reactions.
National Standards:
Context of Lesson:
This is a 90 minute lesson during which students will explore the methods necessary to calculate an empirical formula. They will then be introduced to the methods required to obtain a molecular formula from an empirical formula and be presented with problems that will connect the two concepts. It should be impressed upon students that being able to calculate a molecular formula is a very useful tool when it comes to Chemistry. The lesson can potentially run over into a second 90 minute period depending on how well students are able to comprehend the material.Opportunities to Learn:
Depth of Knowledge
Prerequisite Knowledge
Plans for Differentiating Instruction
Students who may have difficulty with calculating empirical and molecular formulas should be given multiple examples of each to reinforce the concept presented. I am not convinced that this is sufficient. The students who are struggling are not the ones who will do extra work. Will you offer other supports? What about the students who learn quickly?Accommodations and modifications
Environmental factors
Materials
Objectives:
Students will derive the empirical formula and the molecular formula from experimental data. Anything else?Instruction:
Opening:
Students will first be administered a brief quiz (mainly conceptual) on the reading assignment that they were asked to complete for homework at the end of the previous class:The answers to the quiz will be embedded in the lesson and it is not necessary to take separate time to go over the quiz; however, the answers should be pointed out where they appear in the lesson. How will your students know this? What question will you ask to frame the day's activities?
Engagement:
Explain to students that percent composition data can now be used to calculate what we call the empirical formula, which we define as: a formula which gives the lowest whole-number ratio of the atoms of the elements in a compound. From the empirical formula we are able to calculate the molecular formula, which we define as: a simple multiple of the empirical formula. Students should record the following examples into their notes. I find this confusing. Will you use a visual representation to illustrate the difference (and utility) of each type of formula?For example: Dinitrogen tetrahydride, whose molecular formula is N2H4, has an empirical formula of NH2 because this is the simplest ratio of nitrogen to hydrogen in the compound.
Now allow students to investigate: Welders use ethyne (C2H2) in their welding torches. Styrene (C8H8) is used in making polystyrene - which are the little beads used in packaging. Do both of these molecular formulas have the same empirical formula? If so, what is it? Answer: Yes, CH
Now, how do we go about calculating the empirical formula of a compound from the percent composition? Let's look at the following example to answer our question: What is the empirical formula of a compound that is 25.9% N and 74.1% O?
- First, we must understand that if we have 100 g of the compound we will have 25.9 g of N and 74.1 g of O. Does everyone see why? Don't give answers, ask questions. They should be able to do this based on previous instruction, right? If you provide the answer, you are making their previous learning unnecessary.
- Second, convert these values to moles: 25.9 g N x 1 mol N/14.0 g N = 1.85 mol N and 74.1 g O x 1 mol O/16.0 g O = 4.63 mol O
- Third, the correct values can be obtained by dividing both molar quantities (N and O) by the smaller number of moles (in this case the moles of N):
1.85 mol N/1.85 = 1 mol N and 4.63 mol O/1.85 = 2.5 mol O So we have an empirical formula of N1O2.5, is this an acceptable answer? Why or why not? Answer: No, not a whole-number ratio- Fourth, this formula does not represent the lowest whole-number ratio. So what should we do? Answer: multiply each part of the ratio by a number that converts the fraction 2.5 to a whole number. In this case we multiply by 2: 1 mol N x 2 = 2 mol N and 2.5 mol O x 2 = 5 mol O So, we get: N2O5 for our empirical formula.
Is there a way to communicate an overall goal and how the students figure out these steps based on their previous work? The problem is that if you do it, it's just transmission, but if they do it, its knowledge construction.Assign the following practice problem:
Calculate the empirical formula of each compound:
- 94.1% O, 5.9% H
- 67.6% Hg, 10.8% S, 21.6% O
While students are working on these move around the room to monitor their progress. Once they are finished have volunteers put the solutions up on the board and explain their answers.Let's now move on to molecular formulas. Why? Is there a question / puzzle/ challenge you can present to frame this procedure. What is the goal? Why is this important? What are we learning that is new? The molecular formula of a compound is either the same as its experimentally determined empirical formula, or it is a simple whole-number multiple of it. We can determine the molecular formula of a compound if we know two things, what are they? Answer: Its empirical formula and its molar mass. Give students the following practice example: Calculate the molecular formula of the compound whose molar mass is 60.0 g and empirical formula is CH4N.
- First, we determine the molar mass of the empirical formula. For example: CH4N has an empirical formula mass of 30 g
- Second, divide the empirical formula mass into the molar mass: molar mass = 60 g, empirical formula mass = 30 g 60.0 g/30.0 g = 2
- Third, multiply the formula subscripts (from the empirical formula) by this value to get the molecular formula: 2(CH4N) = C2H8N2
Assign the following practice problem:Find the molecular formula of each compound given its empirical formula and molar mass:
Closure:
Ask students the following questions to close the lesson:- How do we calculate empirical formulas? i.e. What steps do we follow?
- Why is it useful to do so?
- How do we calculate molecular formulas? i.e. What steps do we follow?
Provided that students were able to get through the entire lesson, assign section review 7.3 on p. 195 for homework, problems: 39, 41a, 42a, 43. Explain to students that this will be checked and gone over in the beginning of the next class and that shortly after to expect a brief quiz (mainly mathematical ).Good.
Assessment:
During the next class period, after the section review 7.3 homework assignment has been collected, checked, and gone over, administer the following quiz to sumatively assess students understanding on the calculation of empirical and molecular formulas:Reflections
(only done after lesson is enacted)Student Work Sample 1 – Approaching Proficiency:
Student Work Sample 2 – Proficient:
Student Work Sample 3 – Exceeds Proficiency: